1. Field of the Invention
The present invention relates to a method for forming a capacitor of a semiconductor device, and more particularly to such a method by which the capacitor can secure high capacitance without loss of its performance.
2. Description of the Prior Art
It is a well-known fact that cell size grows smaller and smaller as a semiconductor device becomes more highly integrated. The smaller cell size is accompanied with a decrease in capacitor area, which in turn results in a reduction of capacitance. Consequently, the existing structure of a capacitor incurs a difficulty in securing sufficient capacitance necessary for constantly maintaining operational characteristics of a semiconductor device.
In a highly integrated semiconductor device currently being produced in mass quantities, a storage electrode is formed in a variety of 3-dimensional structures, high dielectric constant material is used as material of a dielectric film, or a dielectric film is formed at a thickness as thin as possible in order to secure capacitance over a certain amount necessary for cell operation. These measures are based on the fact that capacitance of a capacitor is proportional to electrode surface area and the dielectric constant of a dielectric film, and is inversely proportional to the distance between electrodes, that is, the thickness of the dielectric film.
For example, storage electrodes having a 3-dimensional structure such as a cylinder structure, a concave structure and a pin structure are intended to increase capacitance by enlarging an electrode surface area, and dielectric films such as Ta2O5, BST and the like are aimed toward increasing capacitance using high dielectric constant material.
On the other hand, since reducing a thickness of a dielectric film has limitations, studies into increasing capacitance are directed toward the enlargement of electrode surface area or the development of a dielectric film made of high dielectric constant material.
The enlargement of an electrode surface area, in particular, also involves difficult processes, and thus most studies are focused on providing high dielectric constant materials.
Although Ta2O5 film has a high dielectric constant, its formation is troublesome due to its material properties, and it reacts with polysilicon of a storage electrode to form an interface film when it is deposited according to a CVD (Chemical Vapor Deposition) method. In addition, O2 loss occurring in a Ta2O5 film causes leakage current.
In conclusion, there is a difficulty in securing sufficient capacitance of a capacitor without loss of its performance through use of the conventional capacitor formation techniques and a single film of Ta2O5.